The present invention is related to an antenna apparatus. More specifically, the present invention is directed to an antenna apparatus which is used as a global positioning system (GPS) antenna, and the like.
As is well known in this technical field, the GPS (Global Positioning System) corresponds to a satellite positioning system with employment of artificial satellites. In the GPS system, electromagnetic waves (GPS signals) having frequencies of approximately 1.57 GHz are received from 4 sets, or more sets of artificial satellites among 24 sets of artificial satellites which are orbiting the earth; positional relationships and temporal errors between these artificial satellites and a moving object are measured based upon the received electromagnetic waves; and a position and an altitude of the moving object on a map can be calculated based upon the basic idea of the trigonometrical survey.
Very recently, the GPS systems are utilized in vehicle navigation systems capable of detecting positions of traveling automobiles, namely, are widely popularized. Vehicle navigation apparatuses are arranged by GPS-purpose antennas, processing apparatuses, display apparatuses, and the like. The GPS-purpose antennas are employed in order to receive GPS signals. The processing apparatuses process the GPS signals received by the GPS-purpose antennases so as to detect present positions of vehicles. The display apparatuses display the present position of the vehicles detected by the processing apparatuses on maps.
On the other hand, in connection with the progress of current compact communication appliances (for example, GPS type vehicle navigation apparatuses, portable type navigation apparatuses, satellite waves receivers, etc.) such as mobile communication appliances, compactnesses and high performance as to antenna apparatuses utilized in these communication appliances are required.
Among these antenna apparatuses, plane type antenna apparatuses (for instance, circular polarized patch antenna, etc.) have such merits that antenna structures thereof are made slim and compact, and these plane type antenna apparatuses can be comparatively easily manufactured in the form of integrated circuits in combination with semiconductor circuits. As a result, these plane type antenna apparatuses are widely employed as antennas utilized in compact communication apparatuses.
As such plane type antenna apparatuses, for instance, antenna apparatuses having the below-mentioned antenna structures are known (refer to, for example, patent document 1): That is, these antenna apparatuses are equipped with circular polarized antenna elements, and circuit boards in which LNA (Low Noise Amplifiers) are formed on rear planes thereof. The circular polarized antenna elements are formed by so-called “patch antenna element.” The circular polarized antenna elements contain dielectric substrates which are manufactured by high dielectric materials such as ceramics. While radiation elements are formed on front surfaces of the dielectric substrates, ground patterns are formed on rear surfaces of the dielectric substrates. Pin holes penetrated from the front surfaces through the rear surfaces of the dielectric substrates are formed in the dielectric substrates. Power feeding pins penetrate through the pin holes, while the power feeding pins connect the radiation elements to the circuit boards. In the plane type antenna apparatuses equipped with the above-described antenna structures, since electric capacitances of antennas can be secured based upon the dielectric substrates made of the high dielectric materials, resonant frequencies of these antennas are lowered, so that the plane type antenna apparatuses can be made compact. In such patch antenna elements, since ground patterns are provided opposite to radiation elements, gains along directions of high elevation angles become high.
In any case, as GPS-purpose antennas, circular polarized antenna elements are used. In other words, GPS signals correspond to circular polarized waves. Further, circular polarized waves are also utilized in the field of ETC signals.
As is well known, an ETC (Electronic Toll Collection) system corresponds to such a system developed as the measure capable of relaxing traffic jams occurred in tool gates where tools as to toll roads (speedways, etc.) are paid. In other words, the above-described ETC system implies such a system that payments of tools are automatically accomplished by utilizing wireless communications at tool gates of speedways. In the above-explained ETC system, ETC signals are communicated in bidirectional communication manners between road-sided antennas equipped in gates installed at tool gates, and gate-passing vehicles equipped with on-vehicle communication appliances having ETC-purpose antennas so as to acquire vehicle information about these gate-passing vehicles, so that the ETC system can execute toll paying business for speedways without stopping the gate-passing vehicles.
There are some possibilities that ETC-purpose antennas are mounted on interior portions of vehicles. For instance, certain ETC-purpose antennas may be set on dashboards under certain angled conditions, or some ETC-purpose antennas may be set on glass of windshields. Also, mounting of ETC-purpose antennas in advance is popularized. That is to say, ETC-purpose antennas are mounted on interior portions of vehicles in factories of vehicle manufactures. In this factory mounting case, there are many cases that these ETC-purpose antennas are installed under such a condition that the ETC-purpose antennas are embedded in rear sides of room mirrors, and embedded under dashboards.
Also, circular polarized plane antennas (curl antenna elements) are known in which circular polarized waves are radiated by elements having spiral shapes.
Referring to FIG. 1 and FIG. 2, a description is made of a conventional cud antenna element.
The curl antenna element 60 is constructed by employing a spiral (whirling) radiation element (antenna element) 62, a ground plate 64 located opposite to the radiation element 61, and a power feeding portion 66 raised from the ground plate 64 along a vertical direction. The ground plate 64 and the spiral-shaped radiation element 62 are arranged substantially parallel to each other. A feeding point 66a of the power feeding portion 66 is provided at a near center of the ground plate 64. It should be noted that an insulator 64a such as a through hole is provided at a center portion of the ground plate 64. As a consequence, the feeding point 66a is not electrically connected to the ground plate 64. In any case, the conventional curl antenna element 60 is an antenna element having a three-dimensional structure.
It should also be noted that curl antenna elements are disclosed in, for instance, a patent document 2 and a patent document 3. While the curl antenna elements disclosed in these patent documents 2 and 3 have three-dimensional structures, since these antenna elements have ground planes which are located opposite to antenna elements in a parallel manner, impedances can be readily matched with each other. Also, the curl antenna elements disclosed in these patent documents 2 and 3 constitute such directional antennas having high gains along zenithal directions thereof due to the ground planes located opposite to the antenna elements.
The above-described patch antenna elements and curl antenna elements are manufactured in the three-dimensional structures, so that thicknesses of antenna elements thereof become bulky. As a result, these patch antenna elements and curl antenna elements can be hardly made thinner. That is to say, in such a case that ground planes located opposite to antenna elements cannot be installed, the patch antenna elements and the curl antenna elements cannot be used as circular polarized antenna elements.
To solve this problem, as the circular polarized antenna elements, film antennas are known which are adhered to windshields of vehicles (refer to, for example, patent document 4). The film antenna disclosed in the patent document 4 is equipped with a single loop-shaped film antenna capable of receiving circular polarized waves on a transparent film. This circular polarized antenna element corresponds to a right-hand polarized antenna equipped with a loop antenna and a non-power feeding element. While edge portions of the loop antenna in the power feeding side are formed in land shapes, these land shapes construct a first power feeding terminal and a second power feeding terminal. The first and second power feeding terminals are connected to first and second connection terminals of a connector which contains a low noise amplifier (LNA) circuit. The connector is connected to a coaxial cable. As a consequence, the first power feeding terminal is connected via the LNA circuit to an inner conductor of the coaxial cable, and the second power feeding terminal is connected to an outer conductor of the coaxial cable.
It should also be noted that as on-vehicle type antenna apparatuses, on-vehicle type digital terrestrial antenna apparatuses used so as to receive digital terrestrial broadcasting waves are known.
[Patent Document 1] Japanese Patent Publication No. 2001-339232 A
[Patent Document 2] Japanese Patent Publication No. 2007-235460 A
[Patent Document 3] Japanese Patent Publication No. 2003-218632 A
[Patent Document 4] Japanese Patent Publication No. 2006-013696 A
In the case that such an antenna apparatus which requires a ground plane located opposite to a radiation element (curl antenna element), as disclosed in the above-described patent documents 2 and 3, is manufactured with employment of a film antenna structure as disclosed in the patent document 4, in the film antenna, the ground plane located opposite to the radiation element cannot be formed, so that impedance matching of the radiation element with respect to the ground plane becomes very difficult.